`!conference plumb-irc` logs spurious errors when run a second time

[squahtx]

When the plumb-irc command fails for a subset of rooms, an admin may want to run it a second time. However, on a second run, it emits spurious errors for rooms that were successfully plumbed the first time, which makes it hard to identify the outstanding failures.

We'd like the command to be idempotent.